NBC famously moved the comedian out of his late weeknight slot once before, believing the time was right then for a change ... and also not wanting to lose another of its then-talents,
Conan O'Brien, who was given the show.

Leno eventually returned, but the departure scenario will be repeated as he leaves "Tonight" a second time Thursday, Feb. 6, paving the way for
Jimmy Fallon to become the host on Feb. 17 (and for the show to move from Los Angeles back to New York, where Fallon is based).

Leno's "Tonight" tenure often has been marked by conversation about more than his performance on the program. Right up to the end, though, NBC has been happy to trumpet that "Tonight" -- along with
"Late Night," which Fallon is leaving (and leaving to fellow
"Saturday Night Live" veteran
Seth Meyers) to move up an hour - continues to "dominate time-slot competition," as recent press releases from the network have phrased it.

If indeed Leno is delivering the biggest "Tonight" ratings the program has had in three years, as NBC maintains, some may question removing him from the show now and replacing him with Fallon. However, the network wants to shore up its demographic stake in the slot, ABC having moved up
"Jimmy Kimmel Live!" at the start of 2013 to go head to head with "Tonight" and CBS'
"Late Show With David Letterman."

Synchronizing Fallon's "Tonight" debut to NBC's Winter Olympics coverage means Leno leaves roughly eight months before his current contract's expiration date. He's getting a nice payout, estimated to be in the neighborhood of $15 million, though he stated to CBS'
"60 Minutes" recently that the timing of the change is "not my decision."

Ten years ago, NBC set a five-year clock on Leno being succeeded by O'Brien ... with Leno playing "good soldier" then by congratulating him on the promotion, not wanting a replay of what happened when Leno was given the show.

His predecessor,
Johnny Carson, had no say over who would inherit "Tonight" from him. As for the sort of feelings that generated, it's noteworthy that Carson later was revealed to have sent jokes to "Tonight" (and Leno) rival Letterman.

Ultimately, Leno and NBC would stay in business after the "Tonight" switch to O'Brien took effect in June 2009. A deal was struck for a weeknight
"Jay Leno Show" in prime time, a way to keep him with the network - as well as a cost-effective move, since that program was significantly less expensive to produce than one of the hour-long drama series that usually fill the last hour of NBC's prime-time lineup.

The ratings impact was pronounced, though, and NBC-affiliated stations quickly grew concerned about its effect on the level of viewership leading into their late local newscasts. Also contributing to the dilemma, O'Brien's "Tonight" ratings were notably lower than Leno's had been, giving CBS and Letterman an advantage NBC obviously wasn't happy about.

A big TV news story of the time was that while O'Brien had a contract to host "Tonight," the agreement didn't mean the show had to be kept at the same time by NBC ... which considered delaying it a half-hour (into the next day, technically) and bringing Leno back for the preceding 30 minutes nightly. O'Brien couldn't abide by that and opted to leave, with NBC giving him a sizable sum for his exit; he would resurface on TBS, where his talk show
"Conan" continues weeknights.

All of the "Tonight Show" hosts mentioned here are members of a very exclusive club, since in becoming the newest one, Fallon will be only the sixth full-time host of the franchise.

Created by legendary TV executive
Sylvester "Pat" Weaver (father of actress
Sigourney Weaver), it originated with
Steve Allen in 1954. After three years,
Jack Paar assumed the hosting, famously walking off the show in 1960 over network censorship of a joke; he returned about a month later and stayed until 1962, when Carson began what would be a 30-year "Tonight" run until Leno got the gig.

And what of Leno after this week? Supposedly, CNN - now overseen by
Jeff Zucker, who was president and CEO of NBC Universal at the time of the Leno/O'Brien switches -- was interested in having him joining the cable news network. However, Zucker said at a January press conference with TV reporters that a Leno-style show is "really not a priority for us at this time."

Other entities are said to have expressed interest in Leno's services, and NBC itself maintains it would like to keep him in the "family," but no decisions about his future were slated to be announced before the end of his "Tonight" tenure.

Fallon surely has big shoes to fill, and that's in terms of the overall history of "The Tonight Show," not just the ratings Leno has achieved during his two stints with it. Meanwhile, the Leno story could prove just as interesting in the coming weeks if a new, television-centric chapter of it is announced.

Given the timing, the iron is hot ... and it's quite possible we may know sooner than later whether Leno strikes.